Size-fractionated phytoplankton biomass and primary production in the Gerlache and south Bransfield Straits (Antarctic Peninsula) in Austral summer 1995-1996

From the beginning of December 1995 to the beginning of February of 1996, 29 stations were visited in the area of Bransfield and Gerlache Straits, and Bellingshausen Sea. Size-fractionated chlorophyll and primary production rates (0.2-2, 2-10 and > 10 mum), as well as phytoplankton species composition were determined. Primary production experiments were carried out following JGOFS protocols and samples incubated from dawn during 24 h. The study area was divided into three main regions: A-Large diatoms region, Gerlache Strait on both cruises, and shelf break of Bellingshausen Sea area during the cruise FRUELA 95, with high values of primary production and predominance of large (> 10 mum) phytosynthetic phytoplankters. Primary production and chlorophyll a (Chl a) concentration averaged, respectively, 2.1 g C m(-2) d(-1) and 165 mg Chl a m(-2) Eighty percent or Chl a and 70% of primary production corresponded to cells larger than 10 mum, such as big chain-forming diatoms, the large flagellate Pyramimonas, and colonies of Phaeocystis. B-Cryptophycean region, Bransfteld Strait and confluence with Gerlache Strait. Characterised by moderate values of primary production of small size organisms. Mean primary production values were ca. 1.0 g C m(-2) d(-1) and 62 mg Chl a m(-2), and the phytoplankton community was formed essentially by Cryptomonas species, Approximately 80% of Chl a and primary production corresponded to less than 10-mum cells. Within this region, a subarea could be distinguished in the central zone of Bransfield Strait, with low phytoplankton biomass (around 20 mg Chl a m(-2)) and primary production (ca 0.6 gC m(-2) d(-1)). In this subarea, the phytoplankton community was dominated by free cells of Phaeocystis and Cryptoinonas; Sixty-five percent of the Chl a and 80% of primary production corresponded to smaller fractions. C-Region of very low phytoplankton biomass, all Bellingshausen Sea during the FRUELA 96 cruise. Primary 2 2 production was around 0.2 g C m(-2) d(-1) and Chl a about 10 mg m(-2). Phytoplankton was dominated by microflagellates (<10 mum), small dinoflagellates (< 20 mum), and very small (< 10 mum) diatoms. More than 70% of the primary production and Chl a corresponded to the smaller than 10-mum fraction. No statistical relationship was found between phytoplankton distribution and hydrography and/or concentrations of dissolved inorganic nutrients for the different regions. The upper mixed layer was always within the photic zone, and levels of nutrients were always high enough to be non-limiting for phytoplankton growth. Grazing pressure as well as sedimentation and respiration losses, in combination with shallow mixed layers, specially in some and very high productive stations of Region A, might be the main factors controlling the phytoplankton distribution in this area of the Antarctic Peninsula. (C) 2001 Elsevier Science Ltd. All rights reserved.